Linking the source, molecular composition, and reactivity of dissolved organic matter in major rivers across the pearl river delta

Dissolved organic matter (DOM) constitutes a substantial portion of the global carbon cycle and serves as the largest reservoir of reactive carbon on Earth. Studying DOM's sources, molecular compositions, and reactivity is central to understanding the fate and environmental effects of organic carbon, yet, links between DOM composition and reactivity remain to be established. In this study, stable carbon isotopic composition and ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) were combined with degradation experiments to comprehensively study DOM's properties in the major rivers across the Pearl River Delta (PRD) in South China. The increasing δ13C-DOC along the Pearl River at the PRD suggests that terrestrial DOM from upstream was gradually diluted by local processes. Although affected by different interaction patterns of multiple sources, a major component of DOM, enriched in CHO and lignins, is molecularly indistinguishable in different rivers (molecular Bray-Curtis dissimilarity of 15%–31%). The photo-degradation removed 34% (PDOC%) of initial DOM within 14 days, twice that by biodegradation (BDOC%). In contrast to earlier research, the distinctive integration of molecular characterization with DOM degradation kinetics indicates that both regional (common shared molecules) and local (partial shared and unique molecules) derived DOM play a collective role in governing the bio-degradation kinetics of DOM, while the photo-degradation kinetics of DOM are barely correlated to molecular composition. The thorough investigation of DOM molecular composition and its correlation with bulk DOM properties in the PRD region, though requiring further investigation into its mechanisms, offers valuable insights into the role of riverine DOM in coastal hypoxia and the global carbon cycle.